Complementary DNAs encoding homologs of the mammalian aquaglyceroporins (termed AQPe) and aquaporin-1 isoforms (termed AQP1) were isolated from the European eel. The AQP amino acid sequences share 35-54% identity with other known human AQPs. Although AQPe mRNA expression was approximately equivalent along the entire length of the gut, AQP1 expression was the highest in the posterior/rectal segment. Seawater (SW) acclimation increased AQP1 mRNA abundance by 5-and 17-fold in the anterior, 14-and 23-fold in the mid-, and 9-and 7-fold in the posterior/rectal gut regions of yellow and silver eels, respectively. SW acclimation had an effect on AQPe mRNA expression only in the midintestine of silver eels, where a small but significant 1.7-fold increase in abundance was measured. Western blots using an eel AQP1-specific antibody identified the presence of a major immunoreactive 28-kDa protein, primarily within the posterior/rectal segment. A 3-wk SW transfer induced an increase in AQP1 protein abundance in all intestinal segments, with the posterior/rectal region still expressing protein levels ϳ40-and 8-fold higher than the anterior and midsegments, respectively. Strong AQP1 immunofluorescence was detected within the vascular endothelium in both freshwater (FW)-and SW-acclimated eels and in the epithelial apical brush border in the posterior/ rectal gut regions of SW-acclimated eels. Cortisol infusion into FW eels had no effect on intestinal AQPe mRNA expression but induced increases in AQP1 mRNA and protein levels. These results provide evidence for the presence of a SW-induced and steroid-regulated AQP water channel pathway within the intestine of the European eel.Anguilla anguilla; gastrointestinal tract; AQP1; AQPe; corticosteroid WHEN FACED WITH EXTERNAL ENVIRONMENTS of varying salinity, euryhaline teleosts, such as the European eel (Anguilla anguilla), have a fundamental osmoregulatory problem: the maintenance of their body fluid composition and osmolality. Osmoregulation is achieved by linked ion and water transport in the gill, kidney, gastrointestinal tract, and urinary bladder (15). The intestine is a major osmoregulatory organ in euryhaline teleost fish, especially when acclimated to seawater (SW). When transferred to the marine environment, euryhaline fish such as the European eel increase their drinking rate by Ͼ10-fold and the ingested SW is mainly desalinated, first within the esophagus, possibly by passive ion diffusion, and thereafter along the entire length of the intestine, by active transport of monovalent ions into the blood. The subsequent water absorption is considered to take place in the intestine by osmotic mechanisms after active absorption of the monovalent ions, the "solute-linked water flow" pathway (52). The magnitude of water fluxes across various parts of the intestine has also been determined, with highest levels occurring in the midregion followed in descending order by the posterior and anterior intestine and rectum (5, 9).Although intestinal salt and water transporters are central t...

These results suggest that SW-acclimation or cortisol infusion induces a down-regulation of renal AQP expression in yellow eels. However, the lower levels of aquaporin expression found within the silver eel kidney were not further reduced by salinity transfer or steroid infusion. These differences in mRNA expression were accompanied by changes in the cellular distribution of the AQP1 protein between vascular endothelial and tubular epithelial cells.